This invention relates to an optical media monitoring device having an optical sensor used in a machine that handles paper and other media to determine the presence or absence of the media. Specifically, it relates to an improvement for detecting degradation of the sensor by dust and adjusting the sensor's sensitivity.
Machines that handle paper and other media customarily detect the presence or absence of media in the machine and the passage of media through the machine by means of optical sensors consisting, for example, of a photoemitter such as a light-emitting diode and a photoreceptor such as a phototransistor. The presence of media between these two components alters the amount of light transmitted, thereby altering the output voltage of the phototransistor. Such an optical sensor tends to accumulate dust during use. The sources of the dust are the ambient air inside the machine and the media itself. The effect of the dust is to degrade the transmittance of light between the light-emitting diode and the phototransistor. It is desirable that such dust degradation be detected by a quick test performable when there is no media present between the light-emitting diode and phototransistor. One such test is the "dust check" performed as follows while the machine is idle: the drive current of the light-emitting diode is limited to reduce the intensity of light emitted; dust degradation is detected if the resulting output voltage of the phototransistor is above a certain level. Another method is called the auto-slice method: the drive current of the light-emitting diode is held fixed, and the reference signal level ("slice level") used to determine whether the media is present or absent is switched according to variations in the output voltage of the phototransistor.
A block diagram of the equipment for the conventional dust check is shown in FIG. 1. The equipment includes a driver circuit 2 that can switch the intensity of the light emitted by the light-emitting diode (LED) 3. The drive circuit 2 comprises two open-collector amplifier stages DRV1 and DRV2, two transistors TR1 and TR2, and two current-limiting resistors R1 and R2 connected to two signals A and B from the sensor controller circuit 1. Signal A produces the normal light intensity; signal B produces the intensity required for the dust check.
When the sensor controller 1 generates the normal-intensity signal A, the transistor TR1 turns on and a current I.sub.1 limited by the current limiting resistor R1 flows to the LED 3. When the dust-check signal B is generated a current I.sub.2 flows to the LED 3. The intensity of light emitted by the LED 3 is proportional to the drive current, and I.sub.2 is set to be smaller than I.sub.1. If the resistance R2 is set to produce the same intensity of received light as when the sensor is degraded by dust, the emitter voltage from the phototransistor 4 can be compared with a reference voltage V.sub.ref in a comparator 5 to produce an on/off signal that indicates whether the sensor is degraded by dust.
This dust check is capable of detecting dust degradation, but it does not enable the sensor to adapt to the degradation so that the machine can continue operating. When dust degradation is detected, the machine must be stopped. In addition, since the slice level V.sub.ref is fixed, the operating margin left in the state of absence of media is reduced, as can be seen by comparing the output voltage from the phototransistor 4 with the slice level V.sub.ref in FIG. 2, where M1 is the margin with no media present before dust degradation, while M2 is the margin after dust degradation. In the partially dust-degraded state there is increased risk that the sensor will erroneously detect media when none is present.
The auto-slice method is illustrated in FIG. 3. The configuration of the LED 3 and phototransistor 4 is similar to that already shown in FIG. 1, but the checking method is different. The intensity of the LED 3 is held fixed and the output voltage of the phototransistor 4 is converted by an analog-to-digital converter 6 to a sensor level signal C which is received and processed digitally by the sensor controller 1. The sensor controller 1 then adjusts the slice level as shown in FIG. 4. S1 in FIG. 4 is the slice level of the signal C in FIG. 3 for determining the presence or absence of media before dust degradation, and S2 is the slice level as changed in response to dust degradation.
By adjusting the slice level to compensate for dust the auto-slice method enables operation to continue despite a certain degree of dust degradation, but the analog-to-digital converter required is relatively expensive. In addition, in a machine with many optical sensors the sensor control circuit must process a large quantity of data, requiring special equipment such as a microprocessor and its peripheral circuits.